Physics

Gravity: A home for the Milky Way

Laniakea – immeasurable heaven in Hawaiian – is the name given by astronomers to the enormous cluster of galaxies that the Milky Way belongs to. The motion of galaxies in this supercluster is determined by gravitational forces.

In this lesson you will investigate the following:

• What is gravity?

• Are astronauts in space really weightless?

• What was Galileo’s experiment?

Prepare for a stellar journey as we travel from the Earth’s surface to the International Space Station and beyond.

This is a print version of an interactive online lesson. To sign up for the real thing or for curriculum details about the lesson go to www.cosmosforschools.com

Introduction: Gravity

Earth,Solar System,Milky Way,The Universe.

Making sure your letter finds its way by adding this to the address is an old children's joke. But maybe it hasn't helped!Scientists have just discovered that it's missing a line!

Our galaxy, the Milky Way, is vast, with over a hundred billion stars. But now astronomers from Hawaii and France have plottedthe even vaster supercluster of galaxies that the Milky Way belongs to. Called Laniakea, its hundred thousand galaxies form a heart-like shape that spans five hundred million light years. 

Actually, we've known for a while that we live in a supercluster, but couldn't tell where it started or finished – it's hard to see theedges from inside. The astronomers had to rely on new technology in telescopes and computing to be able to gather and thenmake sense of the vast amounts of data they needed.

And that data gave them more than just a static picture – it also told them how the galaxies are moving. This turns out to be crucialinformation because there are other superclusters nearby...relatively speaking. With the information about movement thescientists could see that all the galaxies in Laniakea are moving towards one central point. Galaxies just outside Laniakea aremoving towards other points, so they belong to neighbouring superclusters.

So why are the galaxies converging? Gravity. The same force that keeps your pen on the desk is operating across enormousdistances, pulling galaxies together at perhaps the largest scale we can imagine!

Meanwhile, at the cosmic post office, they're breathing a sigh of relief, "Oh, that Milky Way...in Laniakea". Expect some extra lettersas they clear the backlog.

Read or listen to the full Cosmos magazine article here.

Left, the International Space Station (ISS) and right, representatives of the five ISS partner nations on the station. Theyare from Russia, Japan, Belgium (representing the European Union), the United States and Canada.

Question 1

Imagine: Scientific research in the modern world is all about working in teams, often with members spread around the globe. Forexample, the team that discovered the boundaries of Laniakea had members in France and Hawaii and was helped by others inAustralia, Puerto Rico and Chile.

Imagine you are a scientist in an international team, working with others from all continents. List some of the benefits anddifficulties you might encounter working in such a team.

Benefits Difficulties

Gather: Gravity

Laniakea and other superclusters demonstrate gravity at perhaps its most impressive, drawing together whole galaxies – each withbillions of stars – across vast expanses of empty space. What is this mysterious force, and how can we describe it?

What is gravity?

3:15

Question 1

Recall: What phrase is used to explain gravity in the video?

Question 2

Think: Gravitational attraction is a force that can make objectsmove towards one another.

True

False

Question 3

Consider: Is a bird on a branch in South America gravitationallyattracted to a pen in the pocket of an office worker in NewDelhi?

Yes

No

Question 4

Remember: You lift a rock and then release it.

Which of the following is true.

The Earth and rock both move towards each

other, but the Earth moves further.

The rock falls towards the Earth, and the Earth

does not move.

The Earth and rock both move towards each

other, but the rock moves further.

Question 5

Consider: In the video, gravity and magnetism are compared inthe following way:

Gravity and magnetism are not the same, but

behave in a similar way.

Gravity is a form of magnetism.

Magnetism is a form of gravity.

Gravity and magnetism are opposing forces.

Question 6

Select: Which words in the square brackets make the sentences true?

The closer two objects are to each other, the [stronger] [weaker] the gravitational attractionbetween them.

The more mass two objects have, the [stronger] [weaker] the gravitational attraction betweenthem.

As it says in the video, people weigh less on the Moon than on Earth, and you've probably heard too that deep in outer space – far,far away from any other objects – you'd be pretty much weightless. 

A helpful way to think about such effects is in terms of a gravitational field. This refers to the gravitational pull of an object in thespace surrounding it, even if there are no objects in the space for the force to act on. 

Gravitational fields

force, and

the length of the arrows represents the strength of theforce.

This is shown for a spherical object in the diagram to the right.Remember, in reality a gravitational field is 3D, not 2D.

The strength of a gravitational field, at any particular position, is how much gravitational force would act on a mass of 1 kg if it wasplaced at that position. Force is measured in newtons (N), so gravitational field strength is measured in newtons per kilogram(N/kg). It is represented by the letter g.

The diagram below shows g for the Earth's gravitational field as you leave its surface and travel out towards the Moon.

Remember: Objects at any position in the diagram are not just in Earth's gravitational field – they're in the gravitational fields of allthe objects in the Universe! But almost all of those other objects are so small or far away that they have virtually no effect. Theexceptions are the Moon and Sun.

Gravitational field strength: g

Gravitational attraction increases or decreases depending onthe mass of objects and how far apart they are. So,for gravitational fields:

the more massive an object is the greater the strength ofits gravitational field, and

the further away you are from the object the weaker thegravitational field at that point.

We can represent a gravitational field using arrows, where:

the direction of the arrows represents the direction of the

Question 7

Plot: Use the information from the diagram above to complete the graph, plotting the strength of the Earth's gravitational field atincreasing altitudes.

Title

Altitude (km)

0k 2k 4k 6k 8k 10k 12k0

2

4

6

8

1010

Gra

vita

tion

al fi

eld

stre

ngth

(N/k

g)

00 14000

Series 1

0 9.8

281 9

x y

Question 8

Estimate: The Earth's gravitational field strength at its surface is 9.8 N/kg. That means a 1 kg bunch of bananas has a force of 9.8 Nacting on it.

Use the graph to estimate the force of the Earth's gravity on the banana bunch if it was taken 10,000 km (shown as "10k" on thegraph) above the Earth's surface.

The last question illustrates the difference between weight and mass. The terms are used interchangeably in everyday language, buttheir scientific meanings are quite different.

Mass: how much matter an object contains, measured in kilograms.

Weight: the gravitational force on an object, measured in newtons.

The amount of matter in the bananas doesn't change – there's 1 kg of bananas on the Earth's surface and 1 kg of bananas 10,000km up. They'd fill you up just as much wherever you ate them!

But the forces acting on them differ. The gravitational field is stronger on the Earth's surface than at 10,000 km, so the gravitationalforce on the bananas – that is, their weight – is different at the two locations. 

Weight vs. mass

weight (N) = mass (kg) x gravitational field strength (N/kg)

or

W = m x g

Example

What is the weight of a 50 kg girl on the Earth's surface?

Mass of girl = 50 kg

Value of g at Earth's surface = 9.8 N/kg

 W = m x g

 = 50 kg x 9.8 N/kg = 490 N

Question 9

Compute: Calculate the weight of the same 50 kg girl on Mars, Venus and Jupiter. Her weight on the Moon has already beencalculated for you.

We can summarize this in an equation:

Process: Gravity

We've all seen footage of astronauts floating around in space, apparently weightless. We usually think that things have weight ifthey drop to the ground. The heavier something is, the harder it falls.

But are the floating astronauts we see really weightless?

Are astronauts weightless?

3:40

Question 1

Decide: Many people think that astronauts in the InternationalSpace Station are weightless because there's no gravity in space.But without gravity the space station couldn't remain in orbit.

To be really weightless, an astronaut would need to be outsidethe gravitational fields of all planets, stars and other bodies.

True

False

Question 2

Select: Which of the arrows in the diagram below best indicatesthe orbit of the International Space Station?

A

B

C

D

Question 3

Calculate: Remember from earlier in the lesson that:

weight (N) = mass (kg) x gravitational field strength (N/kg)

The International Space Station orbits at an altitude of 370 kmwhere the Earth's gravitational field strength is 8.8 N/kg.

Calculate the weight of an 80 kg astronaut "floating" in thespace station.

Question 4

Reflect: In light of your answer to the previous question, explainwhy the terms "weightless" and "zero gravity" are misleadingdescriptions of astronauts in the space station.

The video explains that the astronauts in the space station aren't floating, they're falling. The reason they appear to be floating isthat the space station is falling too. If you and the room you're in are falling at the same rate then you're never able to catch up tothe floor to stand on it!

But of course the astronauts and the space station aren't falling straight down. They're moving sideways at 28,000 km/h – so fastthat they follow the curvature of the planet and never fall vertically at all. This is just what it means for something to orbit.

Question 5

Projectile motion is what you get when you throw a ball across afootball field. Gravity causes the ball to fall back to the groundalong a curved path.

If you were thrown at the same time as the ball so as to followthe same path (ignoring air resistance) then the ball would seemto float weightless before your eyes.

  True

  False

Question 6

Explain: In all three types of falling the astronauts would appearand feel weightless – so long as they stay above theatmosphere. Suggest a reason why entering the atmospherewould spoil the feeling of weightlessness. 

Did you know?

Because the International Space Station is travelling so fast –nearly 8 km per second – it only takes 90 minutes to circle theEarth.

Astronauts living and working in the space station get to see 16sunrises and sunsets every day!

 

3:17

Question 7

Compare: Although there's still gravity in the space station itdoesn't have its usual effects. List two ways that water behavesdifferently in the "zero gravity" environment of the space stationbased on what you observe in the video.

Question 8

Predict: If you started crying while in the space station whatwould happen to your tears?

Question 9

Design: If you could request an experiment to be conducted in the space station to answer a question about gravity, what would itbe?

Use the project space below to design your experiment, beginning with a clear statement of the question you would like to answeror the hypothesis you would like to test. You can use images, diagrams, etc. to illustrate your ideas.

Apply: Gravity

Experiment: Recreate a famous experiment

Aristotle was a Greek philosopher who lived in the 4th century BCE. He had many theories about the physical world and these, overtime, came to be accepted as beyond challenge. Two that are relevant to this lesson were that the Earth is at the centre of theuniverse – not in the corner of a galaxy in the corner of a supercluster – and that heavier objects fall faster than lighter ones.

Almost 2,000 years later Galileo Galilei, in Pisa, Italy, was applying a new way of going about science. He didn't justbelieve everything that Aristotle had written, but actually conducted observations and experiments to find out!

According to legend Galileo decided to test the theory about falling objects. Fortunately, the perfect resource was right at hand – the Tower of Pisa, 54 metres tall. Galileo climbed the tower and dropped balls of varying mass from the top. And – so the storygoes – they all landed at the same time.

Aristotle had got it wrong.

A short history of dropping things...

Design and carry out an experiment to replicate Galileo's, and present a report of the experiment in the project space below.

It will be best to work in teams, but first, answer the questions below.

Hint: you may be able to use some of the answers in your report.

Have your teacher check a plan of your experiment before carrying it out. This is especially important if you intend climbinganything in order to drop your objects. You also need to be sure the dropped objects do not bounce or shatter when they impactthe ground, harming students recording their fall.

Your task

Safety

Question 1

Consider: In choosing which objects to drop, what considerations will you have to take into account? How will you know what themasses of your objects are?

Hint: would a feather be a suitable object? If not, why not?

Question 2

Engineer: For the experiment to work you have to be sure that you drop the objects at the same time. How will you ensure this? Isthere any way you can check if the objects were released at the same time?

Question 3

Report: Use this space to upload a report of your experiment, including a video of the results. Organize the report under thefollowing titles: 

Background

Aim

Hypothesis

Variables

Materials

Procedure

Results

Discussion

Conclusion

In the discussion section, make sure to identify factors that you would change if you could carry out the experiment again, but withbetter equipment. Why would you make these changes?

3:42

Contrast: Watch Brian Cox carrying out his version of Galileo's experiment below. Your school probably doesn't have equipmentthis good! 

Career: Gravity

Megan Johnson is an astronomer with a special affinity for dwarf galaxies, the little galaxies that may have been the building blocksof our very own Milky Way.

Growing up in a small town in Connecticut, USA, Megan actuallydreamt of becoming a ballerina and joining a dance troupe inNew York. But when a broken leg put an end to her balletcareer, Megan fell back on her other love – mathematics.

After studying and working in finance, Megan happened to readan interview with a young astronomer who had just discovereda whole new spiral arm of the Milky Way. Intrigued by howastronomy seemed to be an application of mathematics, shedecided to study astronomy and has never looked back.

Now Megan works as an astronomer at CSIRO in Sydney,studying dwarf galaxies. She wants to know everything aboutthem – from what they look like to how they evolve and formstars. One of the most interesting things Megan has learnt isthat dwarf galaxies interact with each other more thanpreviously thought. Being so small, dwarf galaxies are helplessagainst the gravitational pull of larger galaxies and are hurledaround space until, occasionally, they slam into each other andmerge. Some astronomers think this can lead to the formationof larger galaxies, like our own Milky Way.

In her quest to survey the small galaxies Megan spends most ofher time at her computer, making sense of data from CSIRO’smany telescopes or writing scientific papers. She also helps outvisiting researchers who come from all over the world to use thetelescopes at CSIRO’s Australian Telescope National Facility.

Despite her astronomical workload, Megan always makes timefor hanging out with her husband and three-year old daughter.

Question 1

Ponder: There are many different types of object in space: stars, galaxies, black holes, pulsars, planets and moons, to name a few.If you were an astronomer, or any other sort of scientist studying things found in places other than on Earth, what would you wantto study, and why? 

Cosmos Lessons team

Lesson authors: Kathryn Grainger and Jim RountreeProfile author: Yi-Di NgIntroduction author: Jim RountreeEditor: Campbell EdgarArt director: Wendy JohnsEducation director: Daniel Pikler

Image credits: iStock, Justus Susterman's portrait of GalileoGalilei (1636),Northrop Grumman Corporation Foundation, Mark A. GarlickVideo credits: TedEd, Veritasium, Canadian Space Agency andBBC on YouTube